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Finding HOT spots New device used after a hot spring caved in the ground By MICHAEL MILSTEIN Gazette Wyoming Bureau YELLOWSTONE NATIONAL PARK - Geologists dream of looking into the Earth, especially in the middle of the bubbling, boiling and spouting landscape of Yellowstone National Park.
 Gazette photos/MICHAEL MILSTEIN Treavor Kendall of the University of Montana, above, uses a ground conductivity meter in hopes of detecting hot spots beneath the Mud Volcano parking lot, where the sinkhole behind him opened up in May. University of Montana researchers have employed a special instrument to do just that. It doesn't see very far into the Earth - maybe 10 feet or a little more depending on the conditions - but they hope that's enough to detect hot spots beneath the parking lot at Yellowstone's Mud Volcano thermal area, where a new hot spring caved in a few parking spaces last month. In a trial run last week, the instrument - called a ground conductivity meter - did detect other hot spots that have not yet revealed themselves on the surface. "It is really a new way of trying to find out what's going on down there without having to excavate huge amounts of earth," said Nancy Hinman, a professor of geology at the University of Montana who is leading the research.
"If it's just this one isolated spot, then we can probably just put a barricade around it," park landscape architect Eleanor Williams said. "But if there is a possibility that we will have this happening in other spots, too, we'd like to know about it." One root of the problem became apparent after Hinman and a group of park staff examined the Mud Volcano area. It seems that the parking lot was built across a gulch that drained many of the area's hot springs, forming a kind of giant dam that has caused spring runoff and silt to pool against the west edge of the parking lot. To get a better look beneath the asphalt, Hinman and Treavor Kendall, a graduate student working with her, used the conductivity meter to compile a subterranean image based upon differences in the electrical conductivity of the ground. Everyone knows water conducts electricity very effectively - that's why you don't want to drop your electric hair drier in the bathtub. And solid materials typically conduct electricity better than air, which is why lightning strikes at trees, tall buildings or mountains that offer it the shortest possible path through the air. So it follows that solid ground will conduct electricity better than a hollow cavity and waterlogged ground will conduct it better than dry patches. Hot water also conducts electricity better than cold water, in part because hot spring water carries tiny particles geologists call "dissolved solids," that help electricity leapfrog through the water. The main element of Hinman's ground conductivity meter, manufactured by Geonics Limited of Canada, is a long pole that generates a weak electrical field. When the pole is held horizontally above the ground like a tightrope walker would hold a pole for balance, the invisible electrical field penetrates the ground. Attached sensors determine how well the field penetrates the ground, producing an immediate measure of the ground's conductivity. Which in turn helps Hinman and Kendall figure out whether the ground in that particular spot is hot or cold, solid or hollow, wet or dry. By measuring the conductivity at many points throughout the parking lot and then plotting the results, they can begin to map out the character of the underlying ground. "Hot thermal water with dissolved solids gives us a very good signal," Hinman said. "It's a starting point for coming up with an idea what the system looks like." While the sinkhole that surfaced in May sits along the west edge of the parking lot, closest to most of the active thermal features, the conductivity survey detected hot spots beneath the opposite side of the parking lot. It's difficult to tell whether the spots are simply hot, soggy sections of ground or new sinkholes waiting to emerge, but clearly there is heat beneath the parking lot that would like to escape the ground. "We did find some other areas under the parking lot where it does appear that heat is building up," Hinman said. A few storm drains covered with metal grates already act as vents in the parking lot, allowing steam to escape from below. It may be that park crews could open a few vents above some of the newly detected hot spots to allow heat there to escape, too, which might prevent it from instead gnawing a cavity beneath the asphalt that could lead to another sinkhole, Hinman said. "It's the buildup of heat that causes problems - if you can somehow give it an outlet, then you might not have problems later on," she said. Hinman will submit a report on the results of the ground conductivity study to park officials, who may then try to incorporate the findings into plans for rehabilitating the parking lot and the main park roads that runs alongside it.
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